ANATOMY. 



sion.which 



<-.-inbe destroyed b\ blowing kir IK 

 ;li. in- -iiiiirm. - . 'Hitiii front thisadlu sion 

 is stronger, and forms a \\ 

 orhicnlus or liguincutum ciliarc. The co- 

 lour of the (.lioroitl coat is a deep brown, 

 approaching to a black, and this colour is 

 derived from a substance called pigmen- 

 tiim niv^ruin, which separates from the 

 choroid by nine er iti'j'i, and dissolves in 

 \v:iU-r so as to render it turbid. 



The inner surfar-: <.f the choroid coat, 

 which is universally coloured by pigmen- 

 tum nigrum in the human subject, is 

 some-times called tunica ruyschiana, as 

 Huysrh endeavoured to prove that it form- 

 ed a distinct membrane from the external 

 part. It is this inner surface that possess- 

 es the hrilliant colours ohservable in ani- 

 iiuds, whence the appellation oftapetum. 

 This surface lies in contact with the retina, 

 but docs not adhere to that membrane. 

 On the front of the eye, however, and be- 

 yond the anterior margin of the retina, the 

 choroid is closely attached by means of 

 numerous and very delicate folds, called 

 the ciliary processes, to the surface of the 

 vitreous humour, round the margin of the 



.Iline lens. 



The iris is a membrane continued trans- 

 across the eye-ball,hehind the cor- 

 nea, and appearing as a continuation of 

 the choroid from the orbiculus ciliaris. 

 The round opening in the front of this 

 membrane is called the pupil ; it allows 

 the passage of the rays of light into the 

 interior oithe eye. This aperture varies 

 in its dimensions according to the quantity 

 of light to which the organ is exposed : a 

 s the pupil to become 

 <'ontracted,in order to exclude aportion of 

 the rays of light which offend the organ. 

 The aperture is dilated in a weak light, to 

 let in as mam ra\ s as possible. Some 

 anatomists have thought properto employ 

 themselves in debating al length, whether 

 'notions arise from a really muscular 

 structure or no; but we believe that they 

 have not \ <. 1 point completely. 



The name of iris was applied to this 

 part, from the daersity of colours obser- 

 vable in it in different individual- 

 it is the colour of this that prodin 

 colour of the eye, in the popular sense of 

 the phrase. There is a remarkable cor- 

 respondence in this point between the 

 skin and hair and the iris. A light com- 

 plexion and hair is accompanied with blue, 

 grey, or the lighter colours of the iris ; 

 while a dark skin and black hair are at- 

 tended with the dark brown iris. 



lu that curious variety of the human 

 race called the Albinos, where the skin 



and hair are of a dead milk-white hue, io 

 of a total absence of the 

 i-ete mucosumor colouring principle, the 

 colouring matter of the iris and choroid is 

 also deficient, and these parts appear red, 

 from the numerous blood-vessels which 



utain. 



The posterior surface of the iris is co- 

 vered by pigmentum nigrum, and is called 

 the u\ 



Under the choroid coat is found a third 

 membrane of the eje-hall, called the reti- 

 na, which is formed by the expansion of 

 the medullary substance of the optic nerve, 

 and forms the immediate organ of vision 

 It is of a yellowish grey colour, and so ex- 

 tremely soft as almost to be lacerated by 

 the slightest touch. Its outer surface is 

 entirely unconnected with the choroid 

 coat; and the inner surface is expanded 

 on the vitreous humour, but not connected 

 to it. It terminates in front by a distinctly 

 defined edge, where the ciliary pr< 

 begin to adhere to the vitreous humour. 

 On the inside of the retina are seen the 

 branches of an artery and vein, which en- 

 ter through the centre of the optic nerve, 

 (arteriaetvenacentralisoculi). The part 

 at which it enters the eye is termed the 

 porus opticus, and is of course insensible ; 

 and hence physiologists have explained 

 the reason why the optic nerve is inserted 

 out of the axis of the eye ; as otli< 

 the axis of vision would have fallen on an 

 insensible part of the retina. 



On the outer, or temporal side of the 

 retina, there is a fold of the membrane of 

 a bright yellow colour, in the recent state, 

 and there is also said to be an aperture. 

 These circumstances were first pointed 

 out by Soemmerring, and have been 

 named after him. 



The vitreous humour occupiesthe great- 

 est share oftheglobe of the e\e. It con- 

 sists of a clear water contained in a cellular 

 substance, which is so perfectly transpa- 

 rent as to resemble pure glass, whence its 

 name is deri\ed. The cell'ilar substance 

 is condensed on the surface into a smooth 

 membrane, called the membrana hyaloi 

 This is marked in front by arirrula 

 series of black radiated lines, caused by 

 the adhesion of the ciliary processes, 

 which, like other parts of the choroid. are 

 covered with pigmentum nigrum 

 these a circular canal runs, named the ra- 

 nal of Petit. 



The crystalline humour or lens is im- 



1 in the front of the vitre. 

 mour. "f a pea, but 



it is much more flattened in form. It isn 

 .\ waxy consistence, softer externally, and 

 growing gradually firmer towards the cen 



